Known apparatus for performing such operations, in which the yarn guides forming the weft transport channel are fixedly connected to the sley and have a "C", or "V" shaped cross section of the channel with an opening towards the reed side, are disclosed for example in German patent documents Nos. 2,622,148 and 2,608,030.
A channel terminating with a rounded portion and in its vertical clearance tapering away from the reed plane, offers an optimum weft thread transport with low energy requirements.
However, the weaving speeds are limited then by the allowable speed of the devices required for forming the shed, so that it is necessary with increasing weaving speeds to reduce the lift of the shed in order to reduce the inertial forces.
This is achieved by reducing the height of the channel perpendicular to the bisecting plane of the upper and lower yarn threads of the shed.
Therefore the known yarn transport channel has a slim "V" or "C" shaped cross section and the legs of the "V" or "C" are nearly parallel to the warp threads when the shed is opened. A slim "V" means that the angle between the upper and lower legs of the "V" is small and so the height of the channel is also small.
The problem of such a channel is the introduction into the warp threads of the lower yarn.
In known apparatus, the upper yarn guide leg enters the group of warp threads of the lower shed, completing this separation via the lower leg of the yarn guide.
The angle between the lower leg of the "V" and the warp threads of the lower shed is important for troublefree penetration of the yarn guide into the shed. In case of a certain lateral displacement of the warp threads, and a lower leg of the "V" being almost parallel to the lower yarn of the shed, there is a danger that the warp threads may not slide down to the bottom of the guides, but will be positioned in crossed manner over the lower leg of the "V".
This lateral displacement may particularly happen in the case of large weaving widths and especially in the vicinity of the stretcher.
Such a warp thread, which is caught on the lower leg of the "V" will cause picking faults because it influences the fluid stream by exposing its filaments to the channel.